nbd: do not leak nbd_trip coroutines when a connection is torn down

Because nbd_client_close removes the I/O handlers for the client
socket, there is no way that any suspended coroutines are restarted.
This will be a problem with the QEMU embedded NBD server, because
we will have a QMP command to forcibly close all connections with
the clients.

Instead, we can exploit the reference counting of NBDClients; shutdown the
client socket, which will make it readable and writeable.  Also call the
close callback, which will release the user's reference.  The coroutines
then will fail and exit cleanly, and release all remaining references,
until the last refcount finally triggers the closure of the client.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

nbd.c

@@ -109,6 +109,7 @@ struct NBDClient {
     Coroutine *send_coroutine;
 
     int nb_requests;
+    bool closing;
 };
 
 /* That's all folks */
@@ -655,19 +656,35 @@ void nbd_client_get(NBDClient *client)
 void nbd_client_put(NBDClient *client)
 {
     if (--client->refcount == 0) {
+        /* The last reference should be dropped by client->close,
+         * which is called by nbd_client_close.
+         */
+        assert(client->closing);
+
+        qemu_set_fd_handler2(client->sock, NULL, NULL, NULL, NULL);
+        close(client->sock);
+        client->sock = -1;
         g_free(client);
     }
 }
 
-static void nbd_client_close(NBDClient *client)
+void nbd_client_close(NBDClient *client)
 {
-    qemu_set_fd_handler2(client->sock, NULL, NULL, NULL, NULL);
-    close(client->sock);
-    client->sock = -1;
+    if (client->closing) {
+        return;
+    }
+
+    client->closing = true;
+
+    /* Force requests to finish.  They will drop their own references,
+     * then we'll close the socket and free the NBDClient.
+     */
+    shutdown(client->sock, 2);
+
+    /* Also tell the client, so that they release their reference.  */
     if (client->close) {
         client->close(client);
     }
-    nbd_client_put(client);
 }
 
 static NBDRequest *nbd_request_get(NBDClient *client)
@@ -810,14 +827,18 @@ out:
 static void nbd_trip(void *opaque)
 {
     NBDClient *client = opaque;
-    NBDRequest *req = nbd_request_get(client);
     NBDExport *exp = client->exp;
+    NBDRequest *req;
     struct nbd_request request;
     struct nbd_reply reply;
     ssize_t ret;
 
     TRACE("Reading request.");
+    if (client->closing) {
+        return;
+    }
 
+    req = nbd_request_get(client);
     ret = nbd_co_receive_request(req, &request);
     if (ret == -EAGAIN) {
         goto done;

nbd.h

@@ -84,6 +84,7 @@ void nbd_export_close(NBDExport *exp);
 
 NBDClient *nbd_client_new(NBDExport *exp, int csock,
                           void (*close)(NBDClient *));
+void nbd_client_close(NBDClient *client);
 void nbd_client_get(NBDClient *client);
 void nbd_client_put(NBDClient *client);